Storage of anthropogenic CO₂ in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO₂ escaping. Although natural CO₂ reservoirs demonstrate that CO₂ may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO₂-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO₂ reservoir-caprock system exposed to CO₂ over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼10⁵ years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO₂.

The pore space of shales and mudrocks ranges from molecular dimensions to micrometers in length scale. This leads to great variation in spatial characteristics across many orders of magnitude, which poses a challenge for the determination of a representative microscopic pore network for such systems. Standard characterization techniques generally provide volume-averaged properties while high-resolution imaging techniques do not assess a representative range of pore sizes because of limitations in the spatial resolution over the field of view. Due to this complexity, open questions remain regarding the role of the pore network in retention and transport processes, which in turn control oil and gas production. Volume-averaged but spatially resolved information is obtained for pores of size from 2 to 150 nm by applying scanning small- and wide-angle X-ray scattering (SAXS and WAXS) microscopy. Scattering patterns are collected in a scanning microscopy mode, such that microvoxels are sampled sequentially, over a total of 2 × 2 mm² raster area on specifically prepared thin sections with a thickness of 10–30 μm. Spatially resolved variations of porosity, pore-size distribution, orientation, as well as mineralogy are derived simultaneously. Aiming at a full characterization of the shale pore network, the measurements and subsequent matrix porosity analysis are integrated in a multiscale imaging workflow involving FIB-SEM, SEM, and μ-CT analysis.